Scale-up of microfluidic devices

ABSTRACT

Parallel uses of microfluidic methods and devices for focusing and/or forming discontinuous sections of similar or dissimilar size in a fluid are described. In some aspects, the present invention relates generally to flow-focusing-type technology, and also to microfluidics, and more particularly parallel use of microfluidic systems arranged to control a dispersed phase within a dispersant, and the size, and size distribution, of a dispersed phase in a multi-phase fluid system, and systems for delivery of fluid components to multiple such devices.

RELATED APPLICATIONS

This application is a continuation of Ser. No. 13/255,342, with a § 371date of Jan. 26, 2012, which is a national stage filing under 35 U.S.C.§ 371 of International Patent Application Serial No. PCT/US2010/000753,filed Mar. 12, 2010, which claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/160,184, filed Mar. 13, 2009, entitled “Scale-upof Microfluidic Devices,” by Romanowsky, et al., and of U.S. ProvisionalPatent Application Ser. No. 61/223,627, filed Jul. 7, 2009, entitled“Scale-up of Microfluidic Devices,” by Romanowsky, et al., all of whichare incorporated herein by reference.

FEDERALLY SPONSORED RESEARCH

This invention was made with government support under Grant No.DMR-0213805 awarded by the National Science Foundation. The governmenthas certain rights in the invention.

FIELD OF THE INVENTION

The present invention relates generally to flow-focusing-typetechnology, and also to microfluidics, and more particularly paralleluse of microfluidic systems arranged to control a dispersed phase withina dispersant, and the size, and size distribution, of a dispersed phasein a multi-phase fluid system, and systems for delivery of fluidcomponents to multiple such devices.

BACKGROUND OF THE INVENTION

The manipulation of fluids to form fluid streams of desiredconfiguration, discontinuous fluid streams, particles, dispersions,etc., for purposes of fluid delivery, product manufacture, analysis, andthe like, is a relatively well-studied art. For example, highlymonodisperse gas bubbles, less than 100 microns in diameter, have beenproduced using a technique referred to as capillary flow focusing. Inthis technique, gas is forced out of a capillary tube into a bath ofliquid, the tube is positioned above a small orifice, and thecontraction flow of the external liquid through this orifice focuses thegas into a thin jet which subsequently breaks into equal-sized bubblesvia a capillary instability. In a related technique, a similararrangement was used to produce liquid droplets in air.

Microfluidics is an area of technology involving the control of fluidflow at a very small scale. Microfluidic devices typically include verysmall channels, within which fluid flows, which can be branched orotherwise arranged to allow fluids to be combined with each other, todivert fluids to different locations, to cause laminar flow betweenfluids, to dilute fluids, and the like. Significant effort has beendirected toward “lab-on-a-chip” microfluidic technology, in whichresearchers seek to carry out known chemical or biological reactions ona very small scale on a “chip,” or microfluidic device. Additionally,new techniques, not necessarily known on the macro scale, are beingdeveloped using microfluidics. Examples of techniques being investigatedor developed at the microfluidic scale include high-throughputscreening, drug delivery, chemical kinetics measurements, combinatorialchemistry (where rapid testing of chemical reactions, chemical affinity,and micro structure formation are desired), as well as the study offundamental questions in the fields of physics, chemistry, andengineering.

The field of dispersions is well-studied. A dispersion (or emulsion) isa mixture of two materials, typically fluids, defined by a mixture of atleast two incompatible (immiscible) materials, one dispersed within theother. That is, one material is broken up into small, isolated regions,or droplets, surrounded by another phase (dispersant, or constantphase), within which the first phase is carried. Examples of dispersionscan be found in many industries including the food and cosmeticindustry. For example, lotions tend to be oils dispersed within awater-based dispersant. In dispersions, control of the size of dropletsof dispersed phase can effect overall product properties, for example,the “feel” of a lotion.

Formation of dispersions typically is carried out in equipment includingmoving parts (e.g., a blender or device similarly designed to break upmaterial), which can be prone to failure and, in many cases, is notsuitable for control of very small dispersed phase droplets.Specifically, traditional industrial processes typically involvemanufacturing equipment built to operate on size scales generallyunsuitable for precise, small dispersion control. Membraneemulsification is one small scale technique using micron-sized pores toform emulsions. However, polydispersity of the dispersed phase can insome cases be limited by the pore sizes of the membrane.

Batch production of discontinuous fluids are prone to difficulties inproduct uniformity. These problems can be compounded for complexstructures such as double emulsions (drops-in-drops) or triple emulsions(drops-in-drops-in-drops). A further difficulty for double or tripleemulsions is poor encapsulation efficiency, where substantial amounts ofthe innermost phase leak out into the outermost phases, which can limitthe usefulness of such emulsions as carriers for valuable or volatilecompounds such as drugs, flavors, or fragrances. Microfluidic devices,by contrast, can produce multiple emulsions with extremely highuniformity and encapsulation efficiency, essentially by regulatingemulsion formation on the individual-droplet level. Such control comesat the cost of forming emulsion droplets essentially one at a time, eachmicrofluidic device producing only small amounts of product on the orderof fractions of a milliliter per hour. The present invention in partinvolves appreciation of the need for scale-up of the products ofmicrofluidic devices.

While many techniques involving control of multi-phase systems exists,there is a need for improvement in control of size of dispersed phase,size range (polydispersity), and other factors.

SUMMARY OF THE INVENTION

The present invention relates generally to flow-focusing-typetechnology, and also to microfluidics, and more particularly paralleluse of microfluidic systems arranged to control a dispersed phase withina dispersant, and the size, and size distribution, of a dispersed phasein a multi-phase fluid system, and systems for delivery of fluidcomponents to multiple such devices. The subject matter of the presentinvention involves, in some cases, interrelated products, alternativesolutions to a particular problem, and/or a plurality of different usesof one or more systems and/or articles.

In one aspect, a method is provided. The methods comprises introducing asubject fluid into an inlet of a channel and expelling separate portionsof the subject fluid from a plurality of microfluidic outlets eachfluidly connected to the inlet, simultaneously, while surrounding atleast one of the separate portions of the subject fluid at least in partwith a dispersing fluid.

In another aspect, a system for forming droplets in microfluidicchannels in parallel is provided. The system comprises a distributionchannel having an inlet fluidly connected to a plurality of microfluidicsubject fluid outlets, each outlet defining a portion of a microfluidicinterconnected region in fluid communication with at least onedispersing fluid channel fluidly connectable to a source of a dispersingfluid.

In another aspect, a system for forming droplets in microfluidicchannels in parallel is provided. The system comprises an interconnectedregion joining a subject fluid channel for carrying a subject fluid, anda dispersing fluid channel for carrying a dispersing fluid, wherein atleast a portion defining an outer wall of the interconnected region anda portion defining an outer wall of the subject fluid channel areportions of a single integral unit.

The subject matter of this application may involve, in some cases,interrelated products, alternative solutions to a particular problem,and/or a plurality of different uses of a single system or article.

Other advantages, features, and uses of the invention will becomeapparent from the following detailed description of non-limitingembodiments of the invention when considered in conjunction with theaccompanying drawings, which are schematic and which are not intended tobe drawn to scale. In the figures, each identical or nearly identicalcomponent that is illustrated in various figures typically isrepresented by a single numeral. For purposes of clarity, not everycomponent is labeled in every figure, nor is every component of eachembodiment of the invention shown where illustration is not necessary toallow those of ordinary skill in the art to understand the invention. Incases where the present specification and a document incorporated byreference include conflicting disclosure, the present specificationshall control.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the present invention will be described byway of example with reference to the accompanying figures, which areschematic and are not intended to be drawn to scale. In the figures,each identical or nearly identical component illustrated is typicallyrepresented by a single numeral. For purposes of clarity, not everycomponent is labeled in every figure, nor is every component of eachembodiment of the invention shown where illustration is not necessary toallow those of ordinary skill in the art to understand the invention. Inthe figures:

FIG. 1 is a schematic illustration of a fluid distribution articleaccording to an embodiment.

FIG. 2 is a schematic illustration of a three-dimensional parallelmicrofluidic device according to an embodiment.

FIG. 3 is a schematic illustration of a one-dimensional parallelmicrofluidic device according to an embodiment.

FIG. 4 is a schematic illustration of a two-dimensional parallelmicrofluidic device according to an embodiment.

FIG. 5 is a schematic illustration of a microfluidic device of theinvention according to an embodiment.

FIG. 6 is a schematic cross-sectional view through line 44 of FIG. 5.

FIG. 7 is a photograph of a two-dimensional parallel microfluidic deviceaccording to an embodiment.

FIG. 8 is a schematic illustration of a parallel microfluidic deviceaccording to an embodiment.

FIG. 9 is a schematic illustration of a parallel microfluidic deviceaccording to an embodiment.

FIG. 10 is a schematic illustration of a parallel microfluidic deviceaccording to an embodiment.

FIG. 11 is a schematic illustration of a parallel microfluidic deviceaccording to an embodiment.

FIG. 12 is a photograph of a microfluidic device according to anembodiment.

DETAILED DESCRIPTION OF THE INVENTION

The following documents are incorporated herein by reference in theirentirety: U.S. Pat. No. 5,512,131, issued Apr. 30, 1996 to Kumar, etal.; International Patent Publication WO 96/29629, published Jun. 26,1996 by Whitesides, et al.; U.S. Pat. No. 6,355,198, issued Mar. 12,2002 to Kim, et al.; International Patent Publication WO 01/89787,published Nov. 29, 2001 by Anderson, et al.; International PatentPublication WO2004/091763, published Oct. 28, 2004 by Link et al.;International Patent Publication WO2004/002627, published Jan. 8, 2004by Stone et al.; International Patent Publication WO2005/021151,published Mar. 10, 2005; WO2007/089541, published Aug. 9, 2007 by Ahn etal.; WO2008/121342, published Oct. 9, 2008 by Chu et al.; WO2006/096571published Sep. 14, 2006 by Weitz et al. Also incorporated herein byreference are U.S. Provisional Patent Application Ser. No. 61/160,020,filed on Mar. 13, 2009, entitled “Controlled Creation of Emulsions,Including Multiple Emulsions,” by Weitz, et al.; U.S. Provisional PatentApplication Ser. No. 61/160,184, filed Mar. 13, 2009, entitled “Scale-upof Microfluidic Devices,” by Romanowsky, et al.; and U.S. ProvisionalPatent Application Ser. No. 61/223,627, filed Jul. 7, 2009, entitled“Scale-up of Microfluidic Devices,” by Romanowsky, et al.

Systems and techniques for parallel use of microfluidic methods anddevices for focusing and/or forming discontinuous sections of similar ordissimilar size in a fluid are provided. In one aspect, a fluiddistribution article is used to distribute fluid from one input to aplurality of outputs. Using the disclosed methods and articles, aplurality of microfluidic devices may be connected in three dimensions.Microfluidic systems and the techniques are described in which, in somecases, it can be important to control back pressure and flow rate suchthat a microfluidic process, such as droplet formation, can be carriedout reproducibly and consistently across a variety of similar oridentical process locations. This is challenging in a microfluidicenvironment and it is not seen where the prior art provides any abilityto achieve this. The present invention does so. In some cases, channeldimensions are chosen that allow pressure variations within paralleldevices to be substantially reduced.

In some embodiments, the present invention involves devices andtechniques associated with manipulation of multiphase materials inparallel. While those of ordinary skill will recognize that any of awide variety of materials including various numbers of phases can bemanipulated in accordance with certain embodiments of the invention,various embodiments of the invention finds use, generally, withtwo-phase systems of incompatible fluids. A “fluid,” as used herein,means any substance which can be urged to flow through devices describedbelow to achieve the benefits discussed herein. Those of ordinary skillin the art will recognize which fluids have viscosity appropriate foruse in accordance with various embodiments of the invention, i.e., whichsubstances are “fluids.” It should be appreciated that a substance maybe a fluid, for purposes of certain embodiments of the invention, underone set of conditions but may, under other conditions, have viscositytoo high for use as a fluid. Where the material or materials behave asfluids under at least one set of conditions compatible with certainembodiments of the invention, they are included as potential materialsfor manipulation.

In one set of embodiments, the present invention involves formation ofdrops of a dispersed phase within a dispersant, of controlled size andsize distribution, in a flow system (preferably a microfluidic system)free of moving parts to create drop formation. That is, at the locationor locations at which drops of desired size are formed, the device isfree of components that move relative to the device as a whole to affectdrop formation or size. For example, where drops of controlled size areformed, they are formed without parts that move relative to other partsof the device that define a channel within the drops flow. This can bereferred to as “passive control” of drop size, or “passive breakup”where a first set of drops are broken up into smaller drops.

Parallel microfluidic devices can be used to produce large-scalequantities of product by integrating many individual devices onto thesame monolithic chip. In some cases, a parallel microfluidic device cangenerate emulsions in quantities of liters per day per integrated chip,or even greater. For example, at least about 200 mL per day perintegrated chip, at least about 1 L per day per integrated chip, atleast about 2 L per day per integrated chip, at least about 5 L per dayper integrated chip, at least about 50 L per day per integrated chip, atleast about 500 L per day per integrated chip, or even more could beproduced.

In some embodiments, parallel scale-up is accompanied by a fluiddistribution article for inputting fluids to, and collecting productfrom, an array of devices. As described in more detail below, the fluiddistribution article and array of devices can be fabricated using knownmethods. The fluid distribution article can be used to operate anarbitrary number of microfluidic devices with a minimum number ofinterfaces to external fluid supplies and collectors, connect a highdensity array of devices, and promote a long functioning lifetime of theintegrated device through system redundancy. Referring now to FIG. 1, aone-dimensional parallel microfluidic system 300 according to oneembodiment of the present invention is illustrated schematically. Afluid distribution article 190 is used to distribute fluid from inputs130 and 140 to a parallel drop formation array 200, and the resultingemulsion formed by the drop formation array exits through output 150.The fluid distribution article allows fluid entering, for example, asingle channel 135 to flow into channel 160 and be distributed to aplurality of channels 165, which enter the drop formation devices insystem 200.

FIG. 2, one embodiment of the present invention, illustratesschematically the assembly of three-dimensional parallel microfluidicsystem 100 of drop formation devices 120. System 100 includes twodimensional arrays 200 of drop formation devices 120. As described inFIG. 1, in this embodiment of the invention a first fluid (i.e., adispersant fluid such as oil) is flowed through input 130 into channel135, a second fluid is flowed through input 140 into channel 145, and anemulsion produced by the interaction of the first fluid and the secondfluid in drop formation devices 120 flow out of the system throughchannel 155 and output 150. Distribution plate 190, which includeschannels 160, 170, and 180, is in a different plane than the plane ofthe two-dimensional drop formation array 200, such that channels 160,170, and 180 are in a different plane than cross-channels 210, 220, and230. Channels 165, 175, and 185, connect channels 160, 170, and 180,respectively, to channels 210, 220, and 230, respectively.

In some embodiments, the fluid distribution article includes one or morelayers of fluidic channels stacked above the layer(s) of microfluidicdevices (FIG. 1). Although “top,” “bottom,” “above,” “below,” etc. areused to define certain portions and perspectives of systems of variousembodiments of the invention, it is to be understood that the systemscan be used in orientations different from those described. The fluiddistribution article can serve one-dimensional (1-D), two-dimensional(2-D), and/or three-dimensional (3-D) arrays of devices in a scalable,parallel configuration. For example, a 1-D linear array of devices maybe served by a single set of fluidic channels as shown in FIG. 3, whichillustrates 1-D array 400 of microfluidic devices 120 in fluidcommunication with channels 210, 220, and 230. In this embodiment,channels 210, 220, and 230 are placed directly over the correspondinginlet or outlet of every device in the array, i.e., channel 210supplying a first fluid to every device through inlets 211, channel 220supplying a second fluid to every device through inlets 221, and channel230 collecting the product from each device from outlets 231. In someembodiments, the fluid distribution article channels have at least oneaperture each (e.g., apertures 212, 222, and 232) on the top side of thechannel for supplying fluid to the corresponding channels and/orcollecting product from the corresponding channels.

A similar design can be used to create a 2-D array of devices with each1-D sub-array served by its own set of distribution channels as shown inFIG. 4, which depicts a 2-D array 500 of microfluidic devices 120, afirst set of distribution channels 210, 220, and 230 in fluidcommunication with each 1-D array of devices, and a second set ofdistribution channels 160, 170, and 180 in fluid communication with thefirst set of distribution channels 210, 220, and 230, respectively.Channels 160, 170, and 180 may have at least two sets of apertures, afirst set of apertures 165, 175, and 185 that connect channels 160, 170,and 180 to channels 210, 220, and 230, respectively, and a second set ofapertures 166, 167, and 168 through which fluid can flow into array 500and/or product can be collected from array 500.

In some embodiments, the distribution channels in each set ofdistribution channels are incorporated into a single layer. Thus, 2-Darray 500 can be constructed by fabricating devices 120 in a firstlayer, distribution channels 210, 220, and 230 in a second layer on topof the first layer, and distribution channels 160, 170, and 180 in athird layer on top of the second layer. Those skilled in the art willrecognize that the order of assembly may be different.

In some cases, a 3-D array is constructed by connecting units of 2-Darrays, as shown in FIG. 2. In some embodiments, a set of distributionchannels (e.g., channels 135, 145, and 155 in FIG. 2) are used tofluidically connect units of 2-D arrays. A 3-D array may be constructedin a variety of conformations, for example by stacking 2-D arrays,placing 2-D arrays side-by-side, etc. As shown in FIG. 2, array 100 maybe operated with a single set of inputs and/or outputs 130, 140, and150.

In some embodiments, distribution channels and devices may beincorporated in a single layer. A non-limiting example is shown in FIG.8, which illustrates an array 600 with two distribution channels 610 and612 that serve two devices 620. In this example, distribution channel610 contains a continuous phase (e.g., an oil) fed by an inlet 614, anddistribution channel 612 contains a dispersed phase (e.g., an aqueoussolution) fed by an inlet 616. The distribution channels feed intodroplet-making devices 620, and the droplets 622 exit the devicesthrough outlets 624. It should be understood that more than two devicesmay be operated using the layout depicted in FIG. 8, for example, byreplicating additional devices 620 side-by-side and extendingdistribution channels 610 and 612 along their respective longitudinalaxes. It should also be understood that arrangements other than lineararrangements may be used. For example, one or more of the devices and/ordistribution channels may be curved or bent. For instance, distributionchannels 610 and 612 and devices 620 may be arranged as shown in FIG. 9,which illustrates two-dimensional array 650.

In another non-limiting example, devices and distribution channelsconstructed for producing droplet-in-droplet emulsions may be fabricatedin a single layer. FIG. 10 depicts one embodiment of this example andshows an array 700 with two distribution channels 710 and 712 servingtwo devices 720. In this example, distribution channel 710 contains acontinuous phase fed by inlet 714, and distribution channel 712 containsa dispersed phase fed by inlet 716. Droplets 722 of the continuous phaseare generated by flowing the continuous phase into a channel 730containing the dispersed phase. Droplets 724, each containing acontinuous phase droplet 722, are generated by flowing droplets 722 intochannels 732. Droplets 724 exit the devices through outlets 726. FIG. 12shows another embodiment of a device constructed for producingdroplet-in-droplet emulsions.

Such droplets may be useful, for example, for producing particles suchas core/shell-type particles. It should be understood that higher orderemulsions (i.e., triple emulsions, quadruple emulsions, etc.) may alsobe generated using designs such as this one. For example, by flowingdroplets 724 into a channel containing another phase instead of intooutlet 726, a triple emulsion may be generated. For instance, anoil-water-oil emulsion may be created by flowing an oil phase into anaqueous phase to generate oil droplets suspended in the aqueous phase,flowing the oil droplets suspended in the aqueous phase into an oilphase to generate an oil-in-water emulsion suspended in the oil phase(i.e., droplets containing an oil droplet suspended in an aqueousdroplet), and flowing the oil-in-water emulsion suspended in the oilphase into an aqueous phase to generate an oil-water-oil emulsion (i.e.,droplets containing an oil droplet suspended in an aqueous dropletsuspended in an oil droplet).

In some embodiments, one or more of the phases, such as the aqueousphase, may contain a surfactant. For example, the aqueous phase maycontain sodium dodecyl sulfate. As discussed herein, the oil phase maybe any suitable material. Non-limiting examples of suitable oil phasesinclude 1-octanol and HFE-7500 oil with 1.8% (by weight) “R22”surfactant as the continuous phase [R22 is the ammonium salt of Krytox®157 FSL oil, a commercially available perfluorinated polyether(Dupont)].

In still another non-limiting example, array 650 shown in FIG. 9 may beoperated differently by changing the direction of flow and the type offluid flowing through the channels, as shown in FIG. 11. FIG. 11illustrates array 800 with devices 820 generating four differentdispersed phases surrounded by a continuous phase. (The four dispersedphases may have the same or different compositions, depending on theapplication.) In this example, distribution channel 610 contains acontinuous phase fed by inlet 614, and collection channel 810,containing the continuous phase fed by channels 840, collects droplets822, 824, 826, and 828, which exit channel 810 through outlet 816.Inlets 830, 832, 834, and 836 each flow a different dispersed phase intocollection channel 810 through channels 840 to generate droplets 822,824, 826, and 828, respectively. This may be used, for example, togenerate libraries of different droplets in parallel. It should beunderstood that inlets 830, 832, 834, and 836 may flow any combinationof the same or different fluids. It should be understood that the arraysshown in FIGS. 8-11 may also be parallelized in three dimensions inother embodiments.

An array, such as depicted in FIG. 11, may lead in certain cases tosubstantial time savings for library generation as compared totraditional methods which involve producing droplets serially in a firststep and then mixing the droplets together in a separate second step.This may be advantageous, for example, when the library contains one ormore sensitive compositions prone to degradation. In some cases, using acommon pressure differential to drive formation of each type of dropletmay improve uniformity in the size of the droplets. In certainembodiments, using a separate inlet channel for each dispersed phase candecrease the potential for contamination of the droplets as compared toinstances, for example, when the same inlet channel is reused to flowdifferent dispersed phases.

The fluid distribution article channels may be fabricated withdimensions (height, width, and/or length) much larger than thedimensions of the device microchannels, which can allow the pressuredrop along the fluid distribution article channels to be essentiallynegligible compared to the pressure drop across each microfluidicdevice. As described in more detail below, such a design can preventhydrodynamic coupling of the devices, ensure their independent andstable performance, and/or partition fluid equally between the devices.Thus, a single set of distribution channels can serve a linear array ofmicrofluidic devices and reduce their interface to a single set ofinlet/outlet apertures without substantially affecting the performanceof the devices.

A fluid distribution article can be used to interface with an array ofmany independent microfluidic devices, thereby allowing an assemblycomprising an arbitrary number of devices to be served with a single setof inlets and outlets. In some embodiments, the methods and articles ofthe present invention allow scaling to at least about 100 devices, atleast about 1,000 devices, at least about 10,000 devices, at least about100,000 devices, or even more.

In some cases, the devices are arranged in a high density array. Forexample, the spacing between devices may be less than 100 microns, lessthan 50 microns, less than 20 microns, less than 10 microns etc. The useof a fluid distribution article also allows denser packing of paralleldevices than can be achieved using single-layer schemes since channelcrossing must be avoided in single-layer schemes.

The total flow rate of fluid entering and/or exiting the assembly may beat least about 100 mL per hour, at least about 1 L per hour, at leastabout 10 L per hour, at least about 100 L per hour, at least about 1000L per hour, or even more.

In one embodiment, an article of the invention is may be constructedcontaining a plurality of devices arranged in three dimensions (e.g. acube-like structure). For example, such an article may contain at least50, 100, 200, 400, 600, or even 10,000 devices. In certain instances, anarticle containing at least such numbers of devices may occupy a volumeof less than 5 cm³. The present invention discloses that a singlemicrofluidic device may have a pressure P and that connecting aplurality of such devices also each having a pressure P using thedisclosed fluid distribution articles does not cause a substantialincrease in the pressure, e.g., the pressure of an article having 10,000devices, each with pressure P may be far less than 10,000×P. In someinstances, the pressure may be less than 10×P, less than 5×P, less than2×P, etc. In certain embodiments, the pressure of a system having aplurality of devices has a pressure essentially equal to P. In thisaspect of the invention, in various embodiments, an article containing anumber of devices as described above, where each device has a pressureP, with a plurality of devices connected as described herein, eachdevice having a pressure P no more than 5% different than any otherpressure P, does not cause an increase in overall pressure of theoverall device more than 25%, 20%, 15%, 10%, 5%, or even 2% more than Pitself.

A further advantage of the present invention is that each device withinan array operates essentially independently from the other devices inthe array. Thus, if a device clogs or otherwise degrades, the otherdevices in the array can continue to operate.

An array of devices connected as described herein using a fluiddistribution article also undergoes a very short turn-on transientbehavior, in contrast to single-layer fan-out schemes that suffer fromlong-lived oscillations before steady-state operation is reached. Forexample, the turn-on transient behavior in a device of the presentinvention may be less than about 10 minutes, less than about 5 minutes,less than about 1 minute, less than about 0.1 minutes, etc.

Pressure oscillation due to hydrodynamic coupling is a common problem inmicrofluidic devices, particularly when elastomeric materials, such asPDMS, are used in the fabrication of the devices. For example, fluidpumped into a channel in an elastomeric microfluidic device can causeexpansion and contraction of the channel thereby introducing a pressurewave in the fluid. In embodiments where the channel serves a pluralityof microfluidic devices, a pressure wave can introduce fluctuations inthe pressure of the fluid feeding into each of the devices connected tothe channel. In some embodiments, the present invention substantiallyavoids these pressure fluctuations by controlling the volume of thechannels feeding the devices. In some cases, a pressure change in adevice may be relieved by the fluid distribution channel therebyessentially preventing the pressure change from affecting anotherdevice. For example, a fluid distribution channel connected to a firstand second device that are in fluid communication with each other canallow the first and second device to be decoupled from each other.

The following tests will be useful for allowing one skilled in the artto design an array of microfluidic devices substantially withouthydrodynamic coupling. For a 1-D array of N essentially identicaldevices connected by a distribution channel, each device has ahydrodynamic resistance value R_(d), and the distribution channel has ahydrodynamic resistance R_(c1) over the distance between adjacentdevices (i.e., the resistance per segment). It is understood that theresistance may be different, but within the same order of magnitude,between inlets and outlets of a device. For example, the resistancebetween the oil inlet and the device outlet may be different incomparison to the resistance between the water inlet and the deviceoutlet. If R_(c1) is much less than R_(d), the fractional difference inflow rates between the first and last devices in the array is less thanN*R_(c1)/R_(d). In some cases, this quantity is maintained below 50%,below 40%, below 30%, below 20%, below 10%, below 1%, below 0.5%, below0.1%, etc.

For a 2-D array of M×N devices, arranged in an M×N grid with M rows ofdevices, each containing N devices and each row being served by its ownset of first-generation linear distribution channels, the hydrodynamicresistance between the inlet of a first-generation distribution channeland the corresponding outlet is approximately R_(d)/N, assuming thatR_(c1) is much less than R_(d). To deliver fluid equally to each of theM rows of devices, the second generation distribution channels shouldhave resistance per segment R_(c2) much less than R_(d)/N. In this case,the fractional difference in flow rates between the first and last rowsof devices is less than M*N*R_(c2)/R_(d). To maintain essentially equalflow per device at the same precision as for the 1-D array, thesecond-generation channels should be designed to have R_(c2)<R_(c1)/M.

Similarly, in a 3-D array of K×M×N devices, arranged in K planes of M×Ngrids, the third generation of channels should have resistance persegment R_(c3)<R_(c2)/K.

Referring now to FIG. 5, one embodiment of the present invention, in theform of a microfluidic system 26, is illustrated schematically incross-section (although it will be understood that a top view of system26, absent top wall 38 of FIG. 6, would appear similar). Although “top”and “bottom” are used to define certain portions and perspectives ofvarious systems of the invention, it is to be understood that thesystems can be used in orientations different from those described. Forreference, it is noted that the system is designed such that fluid flowsoptimally from left to right per the orientation of FIG. 5.

System 26 includes a series of walls defining regions of themicrofluidic system via which the system will be described. Amicrofluidic interconnected region 28 is defined in the system by walls29, and includes an upstream portion 30 and a downstream portion 32,connected to an outlet further downstream which is not shown in FIG. 5.In the embodiment illustrated in FIG. 5, a subject fluid channel 34,defined by side walls 31, is provided within the outer boundaries ofinterconnected region 28. Subject fluid channel 34 has an outlet 37between upstream portion 30 and downstream portion 32 of interconnectedregion 28. The system is thus arranged to deliver a subject fluid fromchannel 34 into the interconnected region between the upstream portionand the downstream portion.

FIG. 6, a cross-sectional illustration through line 4-4 of FIG. 5 shows(in addition to some of the components shown in FIG. 5, such as walls 29and 31) a bottom wall 36 and a top wall 38 which, together with walls 29and 31, defining continuous region 28 (at upstream portion 30 thereof)and subject fluid channel 34. It can be seen that interconnected region28, at upstream portion 30, includes two separate sections, separated bysubject fluid channel 34. The separate sections are interconnectedfurther downstream.

Referring again to FIG. 5, interconnected region 28 includes adimensionally-restricted section 40 formed by extensions 42 extendingfrom side walls 29 into the interconnected region. Fluid flowing fromupstream portion 30 to downstream portion 32 of the interconnectedregion must pass through dimensionally-restricted section 40 in theembodiment illustrated. Outlet 37 of subject fluid channel 34 ispositioned upstream of the dimensionally-restricted section. In theembodiment illustrated, the downstream portion of interconnected region28 has a central axis 44, which is the same as the central axis ofsubject fluid channel 34. That is, the subject fluid channel ispositioned to release subject fluid upstream of thedimensionally-restricted section, and in line with thedimensionally-restricted section. As arranged as shown in FIG. 5,subject fluid channel 34 releases subject fluid into an interior portionof interconnected region 28. That is, the outer boundaries of theinterconnected region are exterior of the outer boundaries of thesubject fluid channel. At the precise point at which fluid flowingdownstream in the interconnected region meets fluid released from thesubject fluid channel, the subject fluid is surrounded at least in partby the fluid in the interconnected region, but is not completelysurrounded by fluid in the interconnected region. Instead, it issurrounded through approximately 50% of its circumference, in theembodiment illustrated. Portions of the circumference of the subjectfluid are constrained by bottom wall 36 and top wall 38.

In the embodiments illustrated, the dimensionally-restricted section isan annular orifice, but it can take any of a varieties of forms. Forexample, it can be elongate, ovoid, square, or the like. Preferably, itis shaped in any way that causes the dispersing fluid to surround andconstrict the cross-sectional shape of the subject fluid. Thedimensionally-restricted section is non-valved in preferred embodiments.That is, it is an orifice that cannot be switched between an open stateand a closed state, and typically is of fixed size.

Although not shown in FIGS. 5 and 6, one or more intermediate fluidchannels can be provided in the arrangement of FIGS. 5 and 6 to providean encapsulating fluid surrounding discontinuous portions of subjectfluid produced by action of the dispersing fluid on the subject fluid.In one embodiment, two intermediate fluid channels are provided, one oneach side of subject fluid channel 34, each with an outlet near theoutlet of the subject fluid channel. In some cases, discontinuoussections of the subject fluid are created by introducing intermediatefluid between the subject fluid and the dispersing fluid, with eachsection surrounded by a shell of the intermediate fluid. In someembodiments, the shell is hardened. The following definitions willassist in understanding certain aspects of the invention. Also included,within the list of definitions, are sets of parameters within whichcertain embodiments of the invention fall.

“Channel,” as used herein, means a feature on or in an article(substrate) that can at least partially confine and direct the flow of afluid, and that has an aspect ratio (length to average cross sectionaldimension) of at least 2:1, more typically at least 3:1, 5:1, or 10:1.The feature can be a groove or other indentation of any cross-sectionalshape (curved, square or rectangular) and can be covered or uncovered.In embodiments where it is completely covered, at least one portion ofthe channel can have a cross-section that is completely enclosed, or theentire channel may be completely enclosed along its entire length withthe exception of its inlet and outlet. An open channel generally willinclude characteristics that facilitate control over fluid transport,e.g., structural characteristics (an elongated indentation) and/orphysical or chemical characteristics (hydrophobicity vs. hydrophilicity)or other characteristics that can exert a force (e.g., a containingforce) on a fluid. The fluid within the channel may partially orcompletely fill the channel. In some cases where an open channel isused, the fluid may be held within the channel, for example, usingsurface tension (i.e., a concave or convex meniscus). The channel may beof any size, for example, having a largest dimension perpendicular tofluid flow of less than about 5 or 2 millimeters, or less than about 1millimeter, or less than about 500 microns, less than about 200 microns,less than about 100 microns, or less than about 50 or 25 microns. Insome cases the dimensions of the channel may be chosen such that fluidis able to freely flow through the reactor. The dimensions of thechannel may also be chosen, for example, to allow a certain volumetricor linear flowrate of fluid in the channel. Of course, the number ofchannels and the shape of the channels can be varied by any method knownto those of ordinary skill in the art. In the embodiments illustrated inthe accompanying figures, all channels are completely enclosed.“Channel”, as used herein, does not include a space created between achannel wall and an obstruction. Instead, obstructions, as definedherein, are understood to be contained within channels. Larger channels,tubes, etc. can be used in microfluidic device for a variety ofpurposes, e.g., to store fluids in bulk and to deliver fluids tocomponents of various embodiments of the invention.

In some, but not all embodiments, all components of the systemsdescribed herein are microfluidic. “Microfluidic”, as used herein,refers to a device, apparatus or system including at least one fluidchannel having a cross-sectional dimension of less than 1 millimeter(mm), and a ratio of length to largest cross-sectional dimension of atleast 3:1, and “microfluidic channel” is a channel meeting thesecriteria. Cross-sectional dimension is measured perpendicular to thedirection of fluid flow. Most fluid channels in certain components ofthe invention have maximum cross-sectional dimensions less than 2millimeters, and preferably 1 millimeter. In one set of embodiments, allfluid channels, at least at regions at which one fluid is dispersed byanother, are microfluidic or of largest cross sectional dimension of nomore than 2 millimeters. In another embodiment, all fluid channelsassociated with fluid dispersion, formed in part by a single component(e.g. an etched substrate or molded unit) are microfluidic or of maximumdimension of 2 millimeters. Of course, larger channels, tubes, etc. canbe used to store fluids in bulk and to deliver fluids to components ofother embodiments of the invention.

A “microfluidic interconnected region,” as used herein, refers to aportion of a device, apparatus or system including two or moremicrofluidic channels in fluid communication.

The “cross-sectional dimension” of the channel is measured perpendicularto the direction of fluid flow. Most fluid channels in components ofvarious embodiments of the invention have maximum cross-sectionaldimensions less than 2 mm, and in some cases, less than 1 mm. In one setof embodiments, all fluid channels are microfluidic or have a largestcross sectional dimension of no more than 2 mm or 1 mm. In anotherembodiment, the fluid channels may be formed in part by a singlecomponent (e.g. an etched substrate or molded unit). Of course, largerchannels, tubes, chambers, reservoirs, etc. can be used to store fluidsin bulk and to deliver fluids to components of various embodiments ofthe invention. In one set of embodiments, the maximum cross-sectionaldimension of all active fluid channels is less than 500 microns, lessthan 200 microns, less than 100 microns, less than 50 microns, or lessthan 25 microns. Devices and systems may include channels havingnon-microfluidic portions as well.

The fluidic droplets within the channels may have a cross-sectionaldimension smaller than about 90% of an average cross-sectional dimensionof the channel, and in certain embodiments, smaller than about 80%,about 70%, about 60%, about 50%, about 40%, about 30%, about 20%, about10%, about 5%, about 3%, about 1%, about 0.5%, about 0.3%, about 0.1%,about 0.05%, about 0.03%, or about 0.01% of the average cross-sectionaldimension of the channel.

As used herein, “integral” means that portions of components are joinedin such a way that they cannot be separated from each other withoutcutting or breaking the components from each other.

A “droplet,” as used herein is an isolated portion of a first fluid thatis completely surrounded by a second fluid. It is to be noted that adroplet is not necessarily spherical, but may assume other shapes aswell, for example, depending on the external environment. In oneembodiment, the droplet has a minimum cross-sectional dimension that issubstantially equal to the largest dimension of the channelperpendicular to fluid flow in which the droplet is located.

The “average diameter” of a population of droplets is the arithmeticaverage of the diameters of the droplets. Those of ordinary skill in theart will be able to determine the average diameter of a population ofdroplets, for example, using laser light scattering or other knowntechniques. The diameter of a droplet, in a non-spherical droplet, isthe mathematically-defined average diameter of the droplet, integratedacross the entire surface. As non-limiting examples, the averagediameter of a droplet may be less than about 1 mm, less than about 500micrometers, less than about 200 micrometers, less than about 100micrometers, less than about 75 micrometers, less than about 50micrometers, less than about 25 micrometers, less than about 10micrometers, or less than about 5 micrometers. The average diameter ofthe droplet may also be at least about 1 micrometer, at least about 2micrometers, at least about 3 micrometers, at least about 5 micrometers,at least about 10 micrometers, at least about 15 micrometers, or atleast about 20 micrometers in certain cases.

As used herein, a “fluid” is given its ordinary meaning, i.e., a liquidor a gas. The fluid may have any suitable viscosity that permits flow.If two or more fluids are present, each fluid may be independentlyselected among essentially any fluids (liquids, gases, and the like) bythose of ordinary skill in the art, by considering the relationshipbetween the fluids. The fluids may each be miscible or immiscible. Forexample, two fluids can be selected to be immiscible within the timeframe of formation of a stream of fluids, or within the time frame ofreaction or interaction. Where the portions remain liquid for asignificant period of time then the fluids should be significantlyimmiscible. Where, after contact and/or formation, the dispersedportions are quickly hardened by polymerization or the like, the fluidsneed not be as immiscible. Those of ordinary skill in the art can selectsuitable miscible or immiscible fluids, using contact angle measurementsor the like, to carry out various techniques of the invention.

As used herein, a first entity is “surrounded” by a second entity if aclosed loop can be drawn around the first entity through only the secondentity. A first entity is “completely surrounded” if closed loops goingthrough only the second entity can be drawn around the first entityregardless of direction. In one aspect, the first entity may be a cell,for example, a cell suspended in media is surrounded by the media. Inanother aspect, the first entity is a particle. In yet another aspect ofthe invention, the entities can both be fluids. For example, ahydrophilic liquid may be suspended in a hydrophobic liquid, ahydrophobic liquid may be suspended in a hydrophilic liquid, a gasbubble may be suspended in a liquid, etc. Typically, a hydrophobicliquid and a hydrophilic liquid are substantially immiscible withrespect to each other, where the hydrophilic liquid has a greateraffinity to water than does the hydrophobic liquid. Examples ofhydrophilic liquids include, but are not limited to, water and otheraqueous solutions comprising water, such as cell or biological media,ethanol, salt solutions, etc. Examples of hydrophobic liquids include,but are not limited to, oils such as hydrocarbons, silicon oils,fluorocarbon oils, organic solvents etc.

The term “determining,” as used herein, generally refers to the analysisor measurement of a species, for example, quantitatively orqualitatively, or the detection of the presence or absence of thespecies. “Determining” may also refer to the analysis or measurement ofan interaction between two or more species, for example, quantitativelyor qualitatively, or by detecting the presence or absence of theinteraction. Example techniques include, but are not limited to,spectroscopy such as infrared, absorption, fluorescence, UV/visible,FTIR (“Fourier Transform Infrared Spectroscopy”), or Raman; gravimetrictechniques; ellipsometry; piezoelectric measurements; immunoassays;electrochemical measurements; optical measurements such as opticaldensity measurements; circular dichroism; light scattering measurementssuch as quasielectric light scattering; polarimetry; refractometry; orturbidity measurements.

The invention, in some aspects, provides for formation of discontinuous,or isolated, regions of a subject fluid in a dispersing fluid, withthese fluids optionally separated by one or more intermediate fluids.These fluids can be selected among essentially any fluids (liquids,gases, and the like) by those of ordinary skill in the art, byconsidering the relationship between the fluids. For example, thesubject fluid and the dispersing fluid are selected to be immisciblewithin the timescale of formation of the dispersed portions. Where thedispersed portions remain liquid for a significant period of time, thefluids should be significantly immiscible. Where, after formation ofdispersed portions, the dispersed portions are quickly hardened bypolymerization or the like, the fluids need not be as immiscible. Thoseof ordinary skill in the art can select suitable immiscible fluids,using contact angle measurements or the like, to carry out varioustechniques of the invention.

In some embodiments, a discontinuous section has a maximum dimension,and the size ratio of the section having the largest maximum dimensionto that having the smallest maximum dimension is at least 10:1, at least25:1, at least 50:1, at least 100:1, etc. The discontinuous sections mayhave a maximum cross-sectional dimension of less than 50 microns, lessthan 25 microns, less than 10 microns, less than 5 microns, less than 1micron, and so on.

Subject fluid dispersion can be controlled by those of ordinary skill inthe art, based on the teachings herein, as well as available teachingsin the field of flow-focusing. Reference can be made, for example, to“Generation of Steady Liquid Microthreads and Micron-Sized MonodispersedSprays and Gas Streams,” Phys. Rev. Lett., 80:2, Jan. 12, 1998,Ganan-Calvo, as well as numerous other texts, for selection of fluids tocarry out the purposes of various embodiments of the invention. As willbe more fully appreciated from the examples below, control of dispersingfluid flow rate, and ratio between the flow rates of dispersing andsubject fluids, can be used to control subject fluid stream and/ordispersion size, and monodispersity versus polydispersity in fluiddispersions. The microfluidic devices of the present invention, coupledwith flow rate and ratio control as taught herein, allow significantlyimproved control and range. The size of the dispersed portion can rangedown to less than one micron in diameter. In some embodiments, the ratioof the flow rate of the subject fluid to the dispersing fluid is lessthan 1:5, less than 1:25, less than 1:50, less than 1:100, less than1:250, less than 1:400, etc. The flow rate of the dispersing fluidwithin a microfluidic channel can be any suitable rate. For example, theflow rate may be between 6×10⁻⁵ and 1×10⁻² milliliters per second,1×10⁻⁴ and 1×10⁻³ milliliters per second, and the like.

Many dispersions have bulk properties (e.g. rheology; how thedispersion(s) flows, and optionally other properties such as opticalproperties, taste, feel, etc., influenced by the dispersion size and thedispersion size distribution. Typical prior art techniques, such asprior art flow focusing techniques, most commonly involve monodispersesystems. The present invention also involves control of conditions thatbidisperse and polydisperse discontinuous section distributions result,and this can be useful when influencing the bulk properties by alteringthe discontinuous size distribution, etc.

The invention, in some embodiments, can be used to form a variety ofdispersed fluid sections or particles for use in medicine (e.g.,pharmaceuticals), skin care products (e.g. lotions, shower gels), foods(e.g. salad dressings, ice cream), ink encapsulation, paint,micro-templating of micro-engineered materials (e.g., photonic crystals,smart materials, etc.), foams, and the like. Highly monodisperse andconcentrated liquid crystal droplets produced according to variousembodiments of the invention can self-organize into two and threedimensional structures, and these can be used in, for example, noveloptical devices.

In some embodiments, a gas-liquid dispersion may be formed to create afoam. As the volume percent of a gas in a gas-liquid dispersionincreases, individual gas bubbles may lose their spherical shape as theyare forced against each other. If constrained by one or more surfaces,these spheres may be compressed to disks, but will typically maintain acircular shape pattern when viewed through the compressing surface.Typically, a dispersion is called a foam when the gas bubbles becomenon-spherical, or polygonal, at higher volume percentages. Although manyfactors, for example, dispersion size, viscosity, and surface tensionmay affect when a foam is formed, in some embodiments, foams form(non-spherical bubbles) when the volume percent of gas in the gas-liquiddispersion exceeds, for example, 75, 80, 85, 90 or 95.

A variety of materials and methods can be used to form components of thesystem, according to one set of embodiments of the present invention. Insome cases various materials selected lend themselves to variousmethods. For example, components of certain embodiments of the inventioncan be formed from solid materials, in which the channels can be formedvia micromachining, film deposition processes such as spin coating andchemical vapor deposition, laser fabrication, photolithographictechniques, etching methods including wet chemical or plasma processes,and the like. See, for example, Angell, et al., Scientific American248:44-55 (1983). In one embodiment, at least a portion of the system isformed of silicon by etching features in a silicon chip. Technology forprecise and efficient fabrication of devices of various embodiments ofthe invention from silicon is known. In another embodiment that section(or other sections) can be formed of a polymer, and can be anelastomeric polymer, or polytetrafluoroethylene (PTFE; Teflon®), or thelike.

Different components can be fabricated of different materials. Forexample, a base portion of a microfluidic device including a bottom walland side walls can be fabricated from an opaque material such as siliconor PDMS, and a top portion, or cover, can be fabricated from atransparent material such as glass or a transparent polymer, forobservation and control of the fluidic process. Components can be coatedso as to expose a desired chemical functionality to fluids that contactinterior channel walls, where base supporting material does not have theprecise, desired functionality. For example, components can befabricated as illustrated, with interior channel walls coated withanother material.

Material used to fabricate various devices of the invention, or materialused to coat interior walls of fluid channels, may desirably be selectedfrom among those materials that will not adversely affect or be affectedby fluid flowing through the device, e.g., material(s) that ischemically inert in the presence of fluids at working temperatures andpressures that are to be used within the device.

In one embodiment, certain components of the invention are fabricatedfrom polymeric and/or flexible and/or elastomeric materials, and can beconveniently formed of a hardenable fluid, facilitating fabrication viamolding (e.g. replica molding, injection molding, cast molding, etc.).The hardenable fluid can be essentially any fluid art that can beinduced to solidify, or that spontaneously solidifies, into a solidcapable of containing and transporting fluids contemplated for use inand with the microfluidic network structures. In one embodiment, thehardenable fluid comprises a polymeric liquid or a liquid polymericprecursor (i.e. a “prepolymer”). Suitable polymeric liquids can include,for example, thermoplastic polymers, thermoset polymers, or mixture ofsuch polymers heated above their melting point; or a solution of one ormore polymers in a suitable solvent, which solution forms a solidpolymeric material upon removal of the solvent, for example, byevaporation. Such polymeric materials, which can be solidified from, forexample, a melt state or by solvent evaporation, are well known to thoseof ordinary skill in the art. A variety polymeric materials, many ofwhich are elastomeric, are suitable, and are also suitable for formingmolds or mold masters, for embodiments where one or both of the moldmasters is composed of an elastomeric material. A non-limiting list ofexamples of such polymers includes polymers of the general classes ofsilicone polymers, epoxy polymers, and acrylate polymers. Epoxy polymersare characterized by the presence of a three-membered cyclic ether groupcommonly referred to as an epoxy group, 1,2-epoxide, or oxirane. Forexample, diglycidyl ethers of bisphenol A can be used, in addition tocompounds based on aromatic amine, triazine, and cycloaliphaticbackbones. Another example includes the well-known Novolac polymers.Examples of silicone elastomers suitable for use according to certainembodiments of the invention include those formed from precursorsincluding the chlorosilanes such as methylchlorosilanes,ethylchlorosilanes, and phenylchlorosilanes, and the like.

Silicone polymers are preferred in one set of embodiments, for example,the silicone elastomer polydimethylsiloxane (PDMS). Exemplarypolydimethylsiloxane polymers include those sold under the trademarkSylgard by Dow Chemical Co., Midland, Mich., and particularly Sylgard182, Sylgard 184, and Sylgard 186. Silicone polymers including PDMS haveseveral beneficial properties simplifying fabrication of themicrofluidic structures of certain embodiments of the invention. Forinstance, such materials are inexpensive, readily available, and can besolidified from a prepolymeric liquid via curing with heat. For example,PDMSs are typically curable by exposure of the prepolymeric liquid totemperatures of about, for example, 65° C. to about 75° C. for exposuretimes of about, for example, 1 hour. Also, silicone polymers, such asPDMS, can be elastomeric and thus may be useful for forming very smallfeatures with relatively high aspect ratios, necessary in certainembodiments of the invention. Flexible (e.g. elastomeric) molds ormasters can be advantageous in this regard.

One advantage of forming structures such as microfluidic structures ofvarious embodiments of the invention from silicone polymers, such asPDMS, is the ability of such polymers to be oxidized, for example byexposure to an oxygen-containing plasma such as an air plasma, so thatthe oxidized structures contain at their surface chemical groups capableof cross-linking to other oxidized silicone polymer surfaces or to theoxidized surfaces of a variety of other polymeric and non-polymericmaterials. Thus, components can be fabricated and then oxidized andessentially irreversibly sealed to other silicone polymer surfaces, orto the surfaces of other substrates reactive with the oxidized siliconepolymer surfaces, without the need for separate adhesives or othersealing means. In most cases, sealing can be completed simply bycontacting an oxidized silicone surface to another surface without theneed to apply auxiliary pressure to form the seal. That is, thepre-oxidized silicone surface acts as a contact adhesive againstsuitable mating surfaces. Specifically, in addition to beingirreversibly sealable to itself, oxidized silicone such as oxidized PDMScan also be sealed irreversibly to a range of oxidized materials otherthan itself including, for example, glass, silicon, silicon oxide,quartz, silicon nitride, polyethylene, polystyrene, glassy carbon, andepoxy polymers, which have been oxidized in a similar fashion to thePDMS surface (for example, via exposure to an oxygen-containing plasma).Oxidation and sealing methods useful in the context of the presentinvention, as well as overall molding techniques, are described in Duffyet al., Rapid Prototyping of Microfluidic Systems andPolydimethylsiloxane, Analytical Chemistry, Vol. 70, pages 474-480,1998, incorporated herein by reference.

Another advantage to forming microfluidic structures of variousembodiments of the invention (or interior, fluid-contacting surfaces)from oxidized silicone polymers is that these surfaces can be much morehydrophilic than the surfaces of typical elastomeric polymers (where ahydrophilic interior surface is desired). Such hydrophilic channelsurfaces can thus be more easily filled and wetted with aqueoussolutions than can structures comprised of typical, unoxidizedelastomeric polymers or other hydrophobic materials. Thus, certaindevices of the invention can be made with surfaces that are morehydrophilic than unoxidized elastomeric polymers.

In some embodiments, it may be desirable to make a channel surfacehydrophobic. One non-limiting method for making a channel surfacehydrophobic comprises contacting the channel surface with an agent thatconfers hydrophobicity to the channel surface. For example, in someembodiments, a channel surface may be contacted (e.g., flushed) withAquapel (a commercial auto glass treatment) (PPG Industries, Pittsburgh,Pa.). In some embodiments, a channel surface contacted with an agentthat confers hydrophobicity may be subsequently purged with air. In someembodiments, the channel may be heated (e.g., baked) to evaporatesolvent that contains the agent that confers hydrophobicity.

Thus, in one aspect of the invention, a surface of a microfluidicchannel may be modified to facilitate the production of emulsions suchas multiple emulsions. In some cases, the surface may be modified bycoating a sol-gel onto at least a portion of a microfluidic channel. Asis known to those of ordinary skill in the art, a sol-gel is a materialthat can be in a sol or a gel state, and typically includes polymers.The gel state typically contains a polymeric network containing a liquidphase, and can be produced from the sol state by removing solvent fromthe sol, e.g., via drying or heating techniques. In some cases, asdiscussed below, the sol may be pretreated before being used, forinstance, by causing some polymerization to occur within the sol.

In some embodiments, the sol-gel coating may be chosen to have certainproperties, for example, having a certain hydrophobicity. The propertiesof the coating may be controlled by controlling the composition of thesol-gel (for example, by using certain materials or polymers within thesol-gel), and/or by modifying the coating, for instance, by exposing thecoating to a polymerization reaction to react a polymer to the sol-gelcoating, as discussed below.

For example, the sol-gel coating may be made more hydrophobic byincorporating a hydrophobic polymer in the sol-gel. For instance, thesol-gel may contain one or more silanes, for example, a fluorosilane(i.e., a silane containing at least one fluorine atom) such asheptadecafluorosilane, or other silanes such as methyltriethoxy silane(MTES) or a silane containing one or more lipid chains, such asoctadecylsilane or other CH₃(CH₂)_(n)— silanes, where n can be anysuitable integer. For instance, n may be greater than 1, 5, or 10, andless than about 20, 25, or 30. The silanes may also optionally includeother groups, such as alkoxide groups, for instance,octadecyltrimethoxysilane. In general, most silanes can be used in thesol-gel, with the particular silane being chosen on the basis of desiredproperties such as hydrophobicity. Other silanes (e.g., having shorteror longer chain lengths) may also be chosen in other embodiments of theinvention, depending on factors such as the relative hydrophobicity orhydrophilicity desired. In some cases, the silanes may contain othergroups, for example, groups such as amines, which would make the sol-gelmore hydrophilic. Non-limiting examples include diamine silane, triaminesilane, or N-[3-(trimethoxysilyl)propyl] ethylene diamine silane. Thesilanes may be reacted to form oligomers or polymers within the sol-gel,and the degree of polymerization (e.g., the lengths of the oligomers orpolymers) may be controlled by controlling the reaction conditions, forexample by controlling the temperature, amount of acid present, or thelike. In some cases, more than one silane may be present in the sol-gel.For instance, the sol-gel may include fluorosilanes to cause theresulting sol-gel to exhibit greater hydrophobicity, and other silanes(or other compounds) that facilitate the production of polymers. In somecases, materials able to produce SiO₂ compounds to facilitatepolymerization may be present, for example, TEOS (tetraethylorthosilicate).

It should be understood that the sol-gel is not limited to containingonly silanes, and other materials may be present in addition to, or inplace of, the silanes. For instance, the coating may include one or moremetal oxides, such as SiO₂, vanadia (V₂O₅), titania (TiO₂), and/oralumina (Al₂O₃).

In some instances, the microfluidic channel is constructed from amaterial suitable to receive the sol-gel, for example, glass, metaloxides, or polymers such as polydimethylsiloxane (PDMS) and othersiloxane polymers. For example, in some cases, the microfluidic channelmay be one in which contains silicon atoms, and in certain instances,the microfluidic channel may be chosen such that it contains silanol(Si—OH) groups, or can be modified to have silanol groups. For instance,the microfluidic channel may be exposed to an oxygen plasma, an oxidant,or a strong acid cause the formation of silanol groups on themicrofluidic channel.

The sol-gel may be present as a coating on the microfluidic channel, andthe coating may have any suitable thickness. For instance, the coatingmay have a thickness of no more than about 100 micrometers, no more thanabout 30 micrometers, no more than about 10 micrometers, no more thanabout 3 micrometers, or no more than about 1 micrometer. Thickercoatings may be desirable in some cases, for instance, in applicationsin which higher chemical resistance is desired. However, thinnercoatings may be desirable in other applications, for instance, withinrelatively small microfluidic channels.

In one set of embodiments, the hydrophobicity of the sol-gel coating canbe controlled, for instance, such that a first portion of the sol-gelcoating is relatively hydrophobic, and a second portion of the sol-gelcoating is relatively hydrophobic. The hydrophobicity of the coating canbe determined using techniques known to those of ordinary skill in theart, for example, using contact angle measurements such as thosediscussed below. For instance, in some cases, a first portion of amicrofluidic channel may have a hydrophobicity that favors an organicsolvent to water, while a second portion may have a hydrophobicity thatfavors water to the organic solvent.

The hydrophobicity of the sol-gel coating can be modified, for instance,by exposing at least a portion of the sol-gel coating to apolymerization reaction to react a polymer to the sol-gel coating. Thepolymer reacted to the sol-gel coating may be any suitable polymer, andmay be chosen to have certain hydrophobicity properties. For instance,the polymer may be chosen to be more hydrophobic or more hydrophilicthan the microfluidic channel and/or the sol-gel coating. As an example,a hydrophilic polymer that could be used is poly(acrylic acid).

The polymer may be added to the sol-gel coating by supplying the polymerin monomeric (or oligomeric) form to the sol-gel coating (e.g., insolution), and causing a polymerization reaction to occur between thepolymer and the sol-gel. For instance, free radical polymerization maybe used to cause bonding of the polymer to the sol-gel coating. In someembodiments, a reaction such as free radical polymerization may beinitiated by exposing the reactants to heat and/or light, such asultraviolet (UV) light, optionally in the presence of a photoinitiatorable to produce free radicals (e.g., via molecular cleavage) uponexposure to light. Those of ordinary skill in the art will be aware ofmany such photoinitiators, many of which are commercially available,such as Irgacur 2959 (Ciba Specialty Chemicals) or2-hydroxy-4-(3-triethoxysilylpropoxy)-diphenylketone (SIH6200.0, ABCRGmbH & Co. KG).

The photoinitiator may be included with the polymer added to the sol-gelcoating, or in some cases, the photoinitiator may be present within thesol-gel coating. For instance, a photoinitiator may be contained withinthe sol-gel coating, and activated upon exposure to light. Thephotoinitiator may also be conjugated or bonded to a component of thesol-gel coating, for example, to a silane. As an example, aphotoinitiator such as Irgacur 2959 may be conjugated to asilane-isocyanate via a urethane bond, where a primary alcohol on thephotoinitiator may participate in nucleophilic addition with theisocyanate group, which may produce a urethane bond.

It should be noted that only a portion of the sol-gel coating may bereacted with a polymer, in some embodiments of the invention. Forinstance, the monomer and/or the photoinitiator may be exposed to only aportion of the microfluidic channel, or the polymerization reaction maybe initiated in only a portion of the microfluidic channel. As aparticular example, a portion of the microfluidic channel may be exposedto light, while other portions are prevented from being exposed tolight, for instance, by the use of masks or filters. Accordingly,different portions of the microfluidic channel may exhibit differenthydrophobicities, as polymerization does not occur everywhere on themicrofluidic channel. As another example, the microfluidic channel maybe exposed to UV light by projecting a de-magnified image of an exposurepattern onto the microfluidic channel. In some cases, small resolutions(e.g., 1 micrometer, or less) may be achieved by projection techniques.

Another aspect of the present invention is generally directed at systemsand methods for coating such a sol-gel onto at least a portion of amicrofluidic channel. In one set of embodiments, a microfluidic channelis exposed to a sol, which is then treated to form a sol-gel coating. Insome cases, the sol can also be pretreated to cause partialpolymerization to occur. Extra sol-gel coating may optionally be removedfrom the microfluidic channel. In some cases, as discussed, a portion ofthe coating may be treated to alter its hydrophobicity (or otherproperties), for instance, by exposing the coating to a solutioncontaining a monomer and/or an oligomer, and causing polymerization ofthe monomer and/or oligomer to occur with the coating.

The sol may be contained within a solvent, which can also contain othercompounds such as photoinitiators including those described above. Insome cases, the sol may also comprise one or more silane compounds. Thesol may be treated to form a gel using any suitable technique, forexample, by removing the solvent using chemical or physical techniques,such as heat. For instance, the sol may be exposed to a temperature ofat least about 150° C., at least about 200° C., or at least about 250°C., which may be used to drive off or vaporize at least some of thesolvent. As a specific example, the sol may be exposed to a hotplate setto reach a temperature of at least about 200° C. or at least about 250°C., and exposure of the sol to the hotplate may cause at least some ofthe solvent to be driven off or vaporized. In some cases, however, thesol-gel reaction may proceed even in the absence of heat, e.g., at roomtemperature. Thus, for instance, the sol may be left alone for a while(e.g., about an hour, about a day, etc.), and/or air or other gases maybe passed over the sol, to allow the sol-gel reaction to proceed.

In some cases, any ungelled sol that is still present may be removedfrom the microfluidic channel. The ungelled sol may be actively removed,e.g., physically, by the application of pressure or the addition of acompound to the microfluidic channel, etc., or the ungelled sol may beremoved passively in some cases. For instance, in some embodiments, asol present within a microfluidic channel may be heated to vaporizesolvent, which builds up in a gaseous state within the microfluidicchannels, thereby increasing pressure within the microfluidic channels.The pressure, in some cases, may be enough to cause at least some of theungelled sol to be removed or “blown” out of the microfluidic channels.

In certain embodiments, the sol is pretreated to cause partialpolymerization to occur, prior to exposure to the microfluidic channel.For instance, the sol may be treated such that partial polymerizationoccurs within the sol. The sol may be treated, for example, by exposingthe sol to an acid or temperatures that are sufficient to cause at leastsome gellation to occur. In some cases, the temperature may be less thanthe temperature the sol will be exposed to when added to themicrofluidic channel. Some polymerization of the sol may occur, but thepolymerization may be stopped before reaching completion, for instance,by reducing the temperature. Thus, within the sol, some oligomers mayform (which may not necessarily be well-characterized in terms oflength), although full polymerization has not yet occurred. Thepartially treated sol may then be added to the microfluidic channel, asdiscussed above.

In certain embodiments, a portion of the coating may be treated to alterits hydrophobicity (or other properties) after the coating has beenintroduced to the microfluidic channel. In some cases, the coating isexposed to a solution containing a monomer and/or an oligomer, which isthen polymerized to bond to the coating, as discussed above. Forinstance, a portion of the coating may be exposed to heat or to lightsuch as ultraviolet right, which may be used to initiate a free radicalpolymerization reaction to cause polymerization to occur. Optionally, aphotoinitiator may be present, e.g., within the sol-gel coating, tofacilitate this reaction.

Additional details of such coatings and other systems may be seen inU.S. Provisional Patent Application Ser. No. 61/040,442, filed Mar. 28,2008, entitled “Surfaces, Including Microfluidic Channels, WithControlled Wetting Properties,” by Abate, et al.; and an InternationalPatent Application filed Feb. 11, 2009, entitled “Surfaces, IncludingMicrofluidic Channels, With Controlled Wetting Properties,” by Abate, etal., each incorporated herein by reference in their entireties.

In one embodiment, a bottom wall is formed of a material different fromone or more side walls or a top wall, or other components. For example,the interior surface of a bottom wall can comprise the surface of asilicon wafer or microchip, or other substrate. Other components can, asdescribed above, be sealed to such alternative substrates. Where it isdesired to seal a component comprising a silicone polymer (e.g. PDMS) toa substrate (bottom wall) of different material, it is preferred thatthe substrate be selected from the group of materials to which oxidizedsilicone polymer is able to irreversibly seal (e.g., glass, silicon,silicon oxide, quartz, silicon nitride, polyethylene, polystyrene, epoxypolymers, and glassy carbon surfaces which have been oxidized).Alternatively, other sealing techniques can be used, as would beapparent to those of ordinary skill in the art, including, but notlimited to, the use of separate adhesives, thermal bonding, solventbonding, ultrasonic welding, etc.

In another embodiment, the present invention generally relates tosystems and methods for creating emulsions, including multipleemulsions. In some cases, emulsions, including multiple emulsions, maybe created through a “triggering” process, where a fluidic droplet orother entity is used to create one or more nestings of dropletscontaining the fluidic droplet or other entity. In such a manner,multiple emulsions may be formed in some cases, e.g., triple emulsions,quadruple emulsions, quintuple emulsions, etc. In certain embodiments, afirst droplet (or other entity) is used to “plug” a channel; fluidpooling behind the droplet pushes the droplet through the channel toform the emulsion. This process may be repeated to create multipleemulsions in some cases. Other aspects of the present inventiongenerally relate to systems for producing such emulsions, methods ofusing such emulsions, methods of promoting such emulsions, or the like.

Thus, in certain embodiments, the present invention generally relates toemulsions, including multiple emulsions, and to methods and apparatusesfor making such emulsions. A “multiple emulsion,” as used herein,describes larger droplets that contain one or more smaller dropletstherein. The larger droplets may be suspended in a third fluid. Incertain embodiments, larger degrees of nesting within the multipleemulsion are possible. For example, an emulsion may contain dropletscontaining smaller droplets therein, where at least some of the smallerdroplets contain even smaller droplets therein, etc. Multiple emulsionscan be useful for encapsulating species such as pharmaceutical agents,cells, chemicals, or the like. As described below, multiple emulsionscan be formed in certain embodiments with generally preciserepeatability.

Fields in which emulsions or multiple emulsions may prove usefulinclude, for example, food, beverage, health and beauty aids, paints andcoatings, and drugs and drug delivery. For instance, a precise quantityof a drug, pharmaceutical, or other agent can be contained within anemulsion, or in some instances, cells can be contained within a droplet,and the cells can be stored and/or delivered. Other species that can bestored and/or delivered include, for example, biochemical species suchas nucleic acids such as siRNA, RNAi and DNA, proteins, peptides, orenzymes, or the like. Additional species that can be incorporated withinan emulsion include, but are not limited to, nanoparticles, quantumdots, fragrances, proteins, indicators, dyes, fluorescent species,chemicals, or the like. An emulsion can also serve as a reaction vesselin certain cases, such as for controlling chemical reactions, or for invitro transcription and translation, e.g., for directed evolutiontechnology.

Using the methods and devices described herein, in some embodiments, anemulsion having a consistent size and/or number of droplets can beproduced, and/or a consistent ratio of size and/or number of outerdroplets to inner droplets (or other such ratios) can be produced forcases involving multiple emulsions. For example, in some cases, a singledroplet within an outer droplet of predictable size can be used toprovide a specific quantity of a drug. In addition, combinations ofcompounds or drugs may be stored, transported, or delivered in adroplet. For instance, hydrophobic and hydrophilic species can bedelivered in a single, multiple emulsion droplet, as the droplet caninclude both hydrophilic and hydrophobic portions. The amount andconcentration of each of these portions can be consistently controlledaccording to certain embodiments of the invention, which can provide fora predictable and consistent ratio of two or more species in a multipleemulsion droplet.

In one aspect, an emulsion may be created through a “triggering”process, where a droplet or other entity is used to create one or morenestings of fluidic droplets containing the droplet or other entity.Other entities besides fluidic droplets, for instance, cells or gelparticles, may also be used in certain embodiments.

More generally, various aspects of the invention are directed to systemsand methods for creating emulsions, including multiple emulsions, usinga process in which a deformable entity, such as a fluidic droplet or agel, at least partially plugs an outlet channel, where the creation of adroplet containing the deformable entity is “triggered” by pushing thedeformable entity into the outlet channel. The outlet channel may be,for instance, a microfluidic channel, as is discussed below. Typically,droplet formation cannot occur without this partial plugging (althoughthere may be a relatively low “error” rate in some embodiments), and sothe formation of the droplet is said to be “triggered” by creating andreleasing the partial plug of the deformable entity into the outletchannel.

As used herein, a “deformable entity” is any entity able to at leastpartially plug an outlet channel, where a carrying fluid containing thedeformable entity cannot flow past the deformable entity into the outletchannel while the deformable entity at least partially plugs the outletchannel. In some cases, the “plugging” may be complete, i.e., viewingthe outlet channel in cross-section, it is not possible for a moleculeof the carrying fluid to flow through the outlet channel withoutcrossing the deformable entity. However, in other cases, the pluggingmay be partial, such that it is theoretically possible for a molecule toenter into the outlet channel without crossing the deformable entity,although the carrying fluid may still be prevented from entering intothe outlet channel due to effects such as viscosity, hydrophobicrepulsion, charge repulsion, or the like.

Other examples may be seen in U.S. Provisional Application No.61/160,020, filed on Mar. 13, 2009, entitled “Controlled Creation ofEmulsions, Including Multiple Emulsions,” by Weitz, et al., incorporatedherein by reference.

The following documents are incorporated herein by reference: U.S.patent application Ser. No. 08/131,841, filed Oct. 4, 1993, entitled“Formation of Microstamped Patterns on Surfaces and DerivativeArticles,” by Kumar, et al., now U.S. Pat. No. 5,512,131, issued Apr.30, 1996; priority to International Patent Application No.PCT/US96/03073, filed Mar. 1, 1996, entitled “Microcontact Printing onSurfaces and Derivative Articles,” by Whitesides, et al., published asWO 96/29629 on Jun. 26, 1996; U.S. patent application Ser. No.09/004,583, filed Jan. 8, 1998, entitled “Method of Forming ArticlesIncluding Waveguides via Capillary Micromolding and MicrotransferMolding,” by Kim, et al., now U.S. Pat. No. 6,355,198, issued Mar. 12,2002; International Patent Application No. PCT/US01/16973, filed May 25,2001, entitled “Microfluidic Systems including Three-DimensionallyArrayed Channel Networks,” by Anderson, et al., published as WO 01/89787on Nov. 29, 2001; S. Provisional Patent Application Ser. No. 60/392,195,filed Jun. 28, 2002, entitled “Multiphase Microfluidic System andMethod,” by Stone, et al.; U.S. Provisional Patent Application Ser. No.60/424,042, filed Nov. 5, 2002, entitled “Method and Apparatus for FluidDispersion,” by Link, et al.; U.S. Provisional Patent Application Ser.No. 60/461,954, filed Apr. 10, 2003, entitled “Formation and Control ofFluidic Species,” by Link, et al.; International Patent Application No.PCT/US03/20542, filed Jun. 30, 2003, entitled “Method and Apparatus forFluid Dispersion,” by Stone, et al., published as WO 2004/002627 on Jan.8, 2004; U.S. Provisional Patent Application Ser. No. 60/498,091, filedAug. 27, 2003, entitled “Electronic Control of Fluidic Species,” byLink, et al.; international Patent Application No. PCT/US2004/010903,filed Apr. 9, 2004, entitled “Formation and Control of Fluidic Species,”by Link, et al., published as WO 2004/091763 on Oct. 28, 2004;International Patent Application No. PCT/US2004/027912, filed Aug. 27,2004, entitled “Electronic Control of Fluidic Species,” by Link, et al.,published as WO 2005/021151 on Mar. 10, 2005; U.S. patent applicationSer. No. 11/024,228, filed Dec. 28, 2004, entitled “Method and Apparatusfor Fluid Dispersion,” by Stone, et al., published as U.S. PatentApplication Publication No. 2005-0172476 on Aug. 11, 2005; U.S.Provisional Patent Application Ser. No. 60/659,045, filed Mar. 4, 2005,entitled “Method and Apparatus for Forming Multiple Emulsions,” byWeitz, et al.; U.S. Provisional Patent Application Ser. No. 60/659,046,filed Mar. 4, 2005, entitled “Systems and Methods of Forming Particles,”by Garstecki, et al.; and U.S. patent application Ser. No. 11/246,911,filed Oct. 7, 2005, entitled “Formation and Control of Fluidic Species,”by Link, et al.

The function and advantage of these and other embodiments of the presentinvention will be more fully understood from the examples below. Thefollowing examples are intended to illustrate the benefits of thepresent invention, but do not exemplify the full scope of the invention.

Example 1

This example demonstrates the fabrication of a parallel drop formationsystem.

An array of microfluidic drop formation devices was fabricated from PDMS(polydimethylsiloxane) using standard multilayer soft lithography. Thefluidic channels were arranged in a PDMS layer to have solid walls andceilings but open floors. Fabrication of the channels was completed bybonding the channel-containing layer to a base of glass or PDMS. Achannel in one layer can be connected to a channel in an adjacent layerby punching a hole in the ceiling of the lower layer channel.

In this example, the bottom-most layer contains an array of microfluidicdevices, which are not connected together within this layer. This devicelayer was plasma bonded to a glass slide coated with a thin layer ofcured PDMS elastomer.

On top of the device layer was bonded a first distribution channel layercontaining an array of fluidic channels, with spacing that matches thatof the inlets in the device layer and with sufficient length to coverthe full row of devices. This single first channel layer constituted thefluid distribution article for a 1-D array of devices.

For a 2-D array of devices, a second channel layer was bonded above thefirst, with a set of channels running perpendicularly to the lower setand with appropriate length and spacing to cover the inlets and outletsof the lower channels as shown in FIG. 7.

To make a 3-D array of devices, several 2-D arrays and fluiddistribution articles were stacked in the following sequence (buildingupwards from a glass slide): glass slide for bottom-most rigid support;solid spacer layer; device layer; fluid distribution article comprisinga first channel layer and a second perpendicular channel layer; secondsequence of solid spacer layer; device layer; fluid distributionarticle; and so on, for an arbitrary number of iterations. The fluiddistribution articles are served by a set of distribution channelsperpendicular to the device layers.

The assembled device array was operated by supplying fluids throughpolyethylene surgical tubing using a syringe pump.

Example 2

This example demonstrates the calculation of channel dimensions for aparallel microfluidic device.

As a sample calculation, the desirable channel dimensions to serve a 5×5array of T-junctions producing simple emulsion droplets was estimatedusing the following equation, which is known in the art:

R=[(12*μ*L)/(w*h ³)]*{[1−[(192/π⁵)*(h/w)]]⁻¹},

where “R” is the resistance in a rectangular microchannel, “μ” is thefluid viscosity, “L” is the channel length, “w” is the channel width,and “h” is the channel height. T-junctions with channel length 4000 μm,width 50 μm, and height 25 μm, have resistance of about 100 kPa*s/μLassuming a viscosity of μ=1 mPa*s. If the first-generation distributionchannels have height 150 μm, width 1500 μm, and the distance betweenadjacent devices is 10,000 μm, then the resistance per segment isR_(c1)=0.2 kPa*s/μL. This affords equal flow division between the 5devices at the 1% accuracy level. For the second-generation distributionchannels, increasing the height to 250 μm and keeping the otherdimensions the same gives R_(c2)=0.04 kPa*s/μL, which again affordsequal flow division at the 1% level.

Example 3

This example demonstrates parallelization of double emulsion formation.

Each dropmaking unit included two sequential cross junctions as shown inFIG. 12. An array of units was molded in one monolithic block of PDMSusing standard soft lithography. Inlet and outlet holes werehand-punched, and plasma bonding was used to seal the microchannels to aglass base plate. Plasma bonding was again used to seal a layer ofdistribution channels onto the array. To make the channel surfaces inthe devices hydrophobic for drop formation, the assembled device wasflushed with Aquapel (a commercial auto glass treatment) and purged withair. The device was baked for several hours to dry the remainingAquapel.

To produce double emulsions, the following fluids were injected throughthe distribution channels: 1-octanol as the innermost phase, water with0.5% (by weight) sodium dodecyl sulfate (“SDS”, a surfactant) as theshell phase, and HFE-7500 oil with 1.8% (by weight) “R22” surfactant asthe continuous phase (R22 is the ammonium salt of Krytox 157 FSL oil, acommercially available perfluorinated polyether). The total flow ratesused were 250 microliters per hour for the innermost phase, 1000microliters per hour for the shell phase, and 4000 microliters per hourfor the continuous phase. Double emulsions of different sizes could beformed by changing the flow rates and/or by using devices with differentsized microchannels. The distribution channels can be adjusted for thesecases, for example by using a calculation as in Example 2. Those ofordinary skill in the art will recognize that auxiliary components, notshown or described in detail herein, are useful in implementing theinvention. For example, sources of various fluids, means for controllingpressures and/or flow rates of these fluids as delivered to channelsshown herein, etc. Those of ordinary skill in the art will readilyenvision a variety of other means and structures for performing thefunctions and/or obtaining the results or advantages described herein,and each of such variations or modifications is deemed to be within thescope of the present invention. More generally, those skilled in the artwould readily appreciate that all parameters, dimensions, materials, andconfigurations described herein are meant to be exemplary and thatactual parameters, dimensions, materials, and configurations will dependupon specific applications for which the teachings of the presentinvention are used. Those skilled in the art will recognize, or be ableto ascertain using no more than routine experimentation, manyequivalents to the specific embodiments of the invention describedherein. It is, therefore, to be understood that the foregoingembodiments are presented by way of example only and that, within thescope of the appended claims and equivalents thereto, the invention maybe practiced otherwise than as specifically described. The presentinvention is directed to each individual feature, system, materialand/or method described herein. In addition, any combination of two ormore such features, systems, materials and/or methods, if such features,systems, materials and/or methods are not mutually inconsistent, isincluded within the scope of the present invention.

In the claims (as well as in the specification above), all transitionalphrases such as “comprising”, “including”, “carrying”, “having”,“containing”, “involving”, “composed of”, “made of”, “formed of” and thelike are to be understood to be open-ended, i.e. to mean including butnot limited to. Only the transitional phrases “consisting of” and“consisting essentially of” shall be closed or semi-closed transitionalphrases, respectively, as set forth in the United States Patent OfficeManual of Patent Examining Procedures, section 2111.03.

What is claimed is: 1-78. (canceled)
 79. A system for forming dropletsin microfluidic channels, comprising: a first plane comprising: aplurality of microfluidic interconnected regions configured for dropletformation, wherein a microfluidic interconnected region of saidplurality of microfluidic interconnected regions is fluidicallyconnected to a subject fluid source, a dispersing fluid source, and afirst droplet outlet channel; and a second plane configured for fluiddistribution, comprising a common droplet outlet channel fluidicallyconnected to said first droplet outlet channel, wherein said secondplane is different from said first plane.
 80. The system of claim 79,wherein said second plane is substantially perpendicular to said firstplane.
 81. The system of claim 79, further comprising a third planecomprising an additional plurality of microfluidic interconnectedregions configured for additional droplet formation, wherein saidadditional plurality of microfluidic interconnected regions isfluidically connected to said subject fluid source, said dispersingfluid source, and a second droplet outlet channel, wherein said seconddroplet outlet channel is fluidically connected to said common dropletoutlet channel, wherein said third plane is different from said firstplane and said second plane.
 82. The system of claim 81, wherein saidfirst plane and said third plane are substantially parallel.
 83. Thesystem of claim 81, wherein a subject fluid distribution channel isfluidically connected to said subject fluid source, wherein said subjectfluid source is in said second plane.
 84. The system of claim 83,wherein said subject fluid distribution channel is in a fourth planethat is different from said first plane, said second plane and saidthird plane.
 85. The system of claim 84, wherein said fourth plane issubstantially parallel to said first plane.
 86. The system of claim 83,wherein said subject fluid distribution channel is fluidically connectedto at least two subject fluid channels.
 87. The system of claim 86,wherein said at least two subject fluid channels are in said firstplane.
 88. The system of claim 87, wherein said at least two subjectfluid channels are fluidically connected to at least two microfluidicinterconnected regions of said plurality of microfluidic interconnectedregions in said first plane.
 89. The system of claim 88, wherein saidplurality of microfluidic interconnected regions in said first plane arepositioned in a first array of interconnected regions and a second arrayof interconnected regions, wherein said first array of interconnectedregions is fluidically connected to said first droplet outlet channelvia a first array droplet outlet channel and said second array ofinterconnected regions is fluidically connected to said first dropletoutlet channel via a second array droplet outlet channel.
 90. The systemof claim 83, further comprising a second subject fluid distributionchannel in said third plane that is fluidically connected to saidsubject fluid source, wherein said subject fluid distribution channeland said second subject fluid distribution channel are fluidicallyconnected to a common subject fluid distribution channel in said secondplane.
 91. The system of claim 81, said dispersing fluid source isfluidically connected to a dispersing fluid distribution channel that isfluidically connected to at least one interconnected region of saidplurality of interconnected regions in said first plane.
 92. The systemof claim 91, wherein said dispersing fluid distribution channel is in afourth plane that is different from said first plane, said second planeand said third plane.
 93. The system of claim 92, wherein said fourthplane is substantially parallel to said first plane.
 94. The system ofclaim 79, wherein said at least two dispersing fluid channels arefluidically connected to at least two microfluidic interconnectedregions of said plurality of microfluidic interconnected regions in saidfirst plane.
 95. The system of claim 87, wherein said dispersing fluidsource comprises a common dispersing fluid distribution channel.
 96. Thesystem of claim 95, further comprising a second dispersing fluiddistribution channel in said third plane, wherein said dispersing fluiddistribution channel and said second dispersing fluid distributionchannel are fluidically connected to said common dispersing fluiddistribution channel in said second plane.
 97. The system of claim 79,wherein said first droplet outlet channel is fluidically connected to atleast two outlet channels.
 98. The system of claim 97, wherein said atleast two outlet channels are in said first plane.
 99. The system ofclaim 79, wherein an aspect ratio of said droplet outlet channel is atleast 2:1.
 100. The system of claim 79, wherein said microfluidicinterconnected region further comprises a dimensionally-restrictedsection formed by extensions extending from an outer wall of saidmicrofluidic interconnected region into said microfluidic interconnectedregion, wherein said dimensionally-restricted section is positionedadjacent to an outlet of a subject fluid channel, wherein said subjectfluid channel is fluidically connected to said subject fluid source.101. The system of claim 100, wherein said microfluidic interconnectedregion is configured to receive a dispersing fluid from said dispersingfluid source and a subject fluid from said subject fluid source, whereinsaid dimensionally-restricted section has a shape which causes saiddispersing fluid to surround and constrict a cross-sectional shape ofsaid subject fluid when said subject fluid exits an outlet of saidsubject fluid channel.
 102. The system of claim 101, wherein saiddimensionally-restricted section is fixed in size.
 103. The system ofclaim 101, wherein said dimensionally-restricted section comprises anannular orifice.
 104. The system of claim 101, wherein said microfluidicinterconnected region is configured such that when said subject fluidexits said outlet of said subject fluid channel, said subject fluid issurrounded by the dispersing fluid at approximately 50% of itscircumference.
 105. The system of claim 79, wherein said common dropletoutlet channel is fluidically connected to a droplet collector.